Myosin, actin and associated cytoskeletal proteins interact to form and maintain the large brush border membrane surface necessary for nutrien absorption in intestinal ipithelial cells. In addition, actomyosin-based movements of microvilli and contractions of the terminal web region of the brush border may be improtant in the absorptive process itself. To define and characterize the molecular basis of these events, regulation of actomyosin ATPase activity will be studied under well-defined conditions using proteins purified from chicken brush borders. Mechansism to be studied for regulation of activity include: a) phosphorylation of myosin heavy and light chain subunits, b) calcium ion binding by myosin, c) calmodulin-dependent calcium ion regulation of myosin kinase(s) and phosphatase(s) and d) calcium ion binding by an actin-associated complex consisting of purified brush border tropomyosin and calmodulin and an affinity-purified tropomyosin- or calmodulin-binding protein. The myosin filament assembly process and its regulation by phosphorylation will also be characterized. These studies should enhance our understanding of myosin function and organization and their regulation in intestinal epithelial and other non-muscle cell.